This application is related to co-pending application Ser. No. 909,973 filed Sept. 22, 1986 for LOW PASS FILTER, now U.S. Pat. No. 4,783,635.
This invention relates generally to low-pass electrical filter circuits, and more particularly the invention relates to a switched capacitor low-pass filter utilizing an active network to improve gain roll-off and DC gain accuracy.
A passive low-pass filter consisting of resistor and capacitor elements exhibits a gain function having significant roll-off; i.e, the filtering of high-frequency signal components occurs generally as a function of frequency. Active filters, or filters including active components, improve the passband flatness of passive circuits, provide DC gain, but introduce DC offset errors in the output signal.
Disclosed in co-pending application Ser. No. 909,973, supra, is a low-pass filter having improved DC offset and DC gain accuracy. As shown in FIG. 1 herein, the filter effectively includes a switched capacitor network in series with a shunt capacitance element of a passive low-pass filter. To obtain a maximally flat network, the cut-off frequency of a low-pass filter should be closely related to the external RC elements, or ##EQU1##
Since the overall filter cutoff frequency depends on the switched capacitor network clock's frequency, when the external or internal clock frequency changes, the filter response loses its shape and may even stop functioning. Therefore, to correctly tune the circuit, one has to choose an external R,C network to satisfy the above relation to each clock frequency.